Structural and optoelectronic properties of silicon germanium alloy thin films deposited by pulsed radio frequency plasma enhanced chemical vapor deposition

Abstract

Hydrogenated amorphous silicon germanium (a-SiGe:H) alloy films are still under study to improve their incorporation into tandem solar cells. In this paper we have investigated the quality of films deposited in a radio frequency powered plasma enhanced chemical vapor deposition unit. Two series of samples were prepared either from a mixture of silane and germane diluted into hydrogen or from the same mixture to which a small amount of argon was added. The applied rf power at 13.56 MHz was either continuous or modulated by a square wave pulse at a frequency of 1356 kHz. Different films were deposited in the two series controlling the time the plasma was “on.” The ratio of the on time to the total period of the modulation, the duty cycle, was varied between 100% (continuous mode) and 50%. The plasma during deposition as well as structural and optoelectronic properties of the resulting films was studied with several techniques. We found an optimum in the transport properties for a duty cycle of 75%: an ambipolar diffusion length of the order of 100 nm for a material presenting a bandgap of 1.44 eV. To explain this promising result, we present a discussion on the growth mechanisms of such layers linking the structural results to the transport data.